1. Technical Field
The invention relates generally to nuclear magnetic resonance (NMR) apparatus and techniques for logging wells. More specifically, the invention relates to a magnet assembly for NMR well logging apparatus.
2. Background Art
NMR is the selective absorption of radio waves by certain atomic nuclei that are immersed in a static magnetic field. The principles of NMR have been used to study the molecular structure of rock formations in oil and gas exploration in a process called well logging. U.S. Pat. No. 5,055,788 issued to Kleinberg et al. discloses an NMR well logging apparatus which includes a magnet array disposed in a tool body and an RF antenna mounted in a recess external to the tool body. The magnet array includes three permanent bar magnets mounted in parallel to each other within the tool body. The magnet array produces a static magnetic field in all regions surrounding the tool, and the antenna produces an oscillating magnetic field which is superposed on the static magnetic field. The volume of investigation, or resonant volume, is an elongated region directly in front of the RF antenna in which the static magnetic field produced by the magnetic array has a saddle point, that is, the region in which the static magnetic field is substantially homogenous and the spatial gradient of the static magnetic field is approximately zero. The tool makes measurements by magnetically tipping the nuclear spins of particles in the formation with a pulse of the oscillating magnetic field, and then detecting the precession of the tipped particles in the static, homogeneous field within the volume of investigation. It should be noted that the depth of investigation is related to the position of the saddle point of the static magnetic field. The further away the saddle point is from the tool face, the greater the depth of investigation, but this is usually at the cost of signal-to-noise ratio.
One aspect of the invention is a magnet assembly for nuclear magnetic well logging apparatus which comprises two diametrically opposed magnets which produce a static and substantially homogeneous magnetic field. Each magnet has a magnet axis and is partitioned into a first and a second magnet segment by a plane substantially parallel to the magnet axis.